This invention relates to lithographic printing. In particular, this invention relates to methods for imaging imageable elements useful as lithographic printing plate precursors either thermally or with ultraviolet radiation and developing them in the same developer.
In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plates, also called printing plate precursors, typically comprise a top layer applied over the surface of a hydrophilic substrate. The top layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
If after exposure to radiation, the imaged regions are removed in the developing process, revealing the underlying hydrophilic surface of the substrate, the plate is called a positive-working printing plate. Conversely, if the unimaged regions are removed by the developing process and the imaged regions remain, the plate is called a negative-working plate. In each instance, the regions of the radiation-sensitive layer (i.e., the image areas) that remain repel water and accept ink, and the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution.
Multi-layer positive-working imageable elements for the preparation of lithographic printing plates comprise at least two layers, an underlayer and a top layer, over a substrate with a hydrophilic surface. Two types of imaging are typically used. An element may imaged through a negative with ultraviolet radiation. Alternatively, an element may be imaged directly with an infrared laser, obviating the need for exposure through a negative.
To obtain a printing plate with imagewise distribution of printable regions, it is necessary to remove the imaged regions of the imageable element by contacting the imaged imageable element with a suitable developer. Ultraviolet imaging and thermal imaging require different types of imageable elements and different types of developers. Consequently, a user that uses both ultraviolet and thermal imaging must maintain an inventory of both types of imageable elements and must have two types of developers. Thus, a need exists for a method for forming printing plates using either ultraviolet or thermal imaging that requires only one type of imageable element and one type of developer.
The invention is a method of preparing images useful as lithographic printing plates using either ultraviolet or thermal imaging, in which the same imageable element and the same developer can be used regardless of the type imaging. The method comprises the steps of:
(a) imaging an imageable element either thermally or with ultraviolet radiation and producing an imaged imageable element comprising imaged and unimaged regions;
(b) developing the imaged imageable element with a developer and removing the imaged regions;
in which:
the imageable element comprises, in order:
a top layer;
an underlayer; and
a substrate;
the underlayer comprises a first polymeric material;
the underlayer is soluble in or removable by the developer;
the top layer is ink receptive;
the top layer is insoluble in the developer prior to imaging;
the top layer comprises a phenolic polymer and a compound that comprises an o-benzoquinonediazide moiety or an o-diazonaphthoquinone moiety; a phenolic polymer that comprises the o-benzoquinonediazide moiety or the o-diazonaphthoquinone moiety; or a mixture thereof; and
the developer is an about 20:80 to about 80:20 mixture of:
(1) an aqueous alkaline developer that has a pH greater than 11 and comprises about 2 wt % to about 8 wt % of an alkali metal silicate, based on the weight of the aqueous alkaline developer, and
(2) a solvent based developer that comprises about 0.5 wt % to about 15 wt % of an organic solvent or mixture of organic solvents, based on the weight of the solvent based developer.
The resulting imaged and developed imageable elements are useful as lithographic printing plates.